500 mw generator
DESCRIPTION
Description of 500mw GeneratorTRANSCRIPT
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19. Design And Constructional Feature Of 500 MW Generator INTRODUCTION Turbo generator for 500MW units are two pole water and hydrogen gas cooled
generator. The primary water flows through the stator winding and picks up heat losses
arising in generator directly, which are transferred to the secondary cooling water. Core
losses, windage losses and rotor winding losses are picked up by circulating hydrogen
and transferred to Hydrogen cooling water in gas coolers. The stator frame is designed to
withstand the impact due to possible explosions inside. A brushless excitation system with
rotating diodes is provided to excite the machine.
Generator a. Type : THDF 115/59
B. Cooling Stator Winding : Directly Water Cooled
c. Stator core & Rotor : Directly Hydrogen Cooled
d. Apparent Power : 588 MVA
e. Active Power : 500 MW
f. Power Factor : 0.85 (lag)
g. Terminal Voltage : 21 KV
h. Stator Current : 16200 Amps.
i. Hydrogen Pressure : 4 Kg/cm 2
j. Short Circuit Ratio : 0.48
k. Class and Type of Insulation : Miscalastic
The two-pole generator uses direct water cooling for the stator winding, phase
connectors and bushings and direct hydrogen cooling for the rotor winding. The losses 227
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in the remaining generator components, such as iron losses, windage losses and stray
losses, are also dissipated through hydrogen.
The generator frame is pressure-resistant and gas tight equipped with one stator end
shield on each side. The hydrogen coolers are arranged vertically inside the turbine end
stator end shield.
The generator consists of the following components
* Stator
Stator frame
End shields
Stator core Stator winding
* Rotor
Rotor shaft Rotor winding
Rotor retaining rings
Field -connections
* Hydrogen Coolers
* Bearings
* Shaft Seals
The following additional auxiliaries are required for generator operation
* Oil system
* Gas system
* Primary water system
* Excitation system
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STATOR
The 3 Armature winding of the generator is housed inside the stator which comprises
of a stator frame, end shields and the bushing compartment. Since an explosive gas
Hydrogen is used, the frame is made rigid enough to withstand a pressure of 10 bar.
The lower part of the frame near the exciter end houses the terminal box. The phase
and neutral leads of the 3 stator winding are brought out of the generator through six
bushings located in the terminal box. The hydrogen coolers are arranged vertically in
the turbine side stator end shield. The stator end shields contain the shaft seal and
bearing components.
STATOR FRAME
The stator frame consist of a cylindrical center section and two end shields which are gas-tight and pressure resistant.
The stator end shields are joined and sealed to the stator frame with an 0-ring and
bolted flange connections. The stator frame accommodates the electrically active parts
of the stator. i.e., the stator core and stator windings. Both the gas ducts and a large
number of welded circular ribs provide for the rigidity of the stator frame. Ring-shaped
supports for resilient core suspension are arranged between the circular ribs. The
generator cooler is sub-divided into cooler sections arranged vertically in the turbine
side stator end shield. In addition, the stator end shields contain the shaft seal and
bearing components. Feet are welded to the stator frame end shields to support the
stator on the foundation. The stator is firmly connected to the foundation with anchor
bolts through the feet. 229
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STATOR CORE
The staler core is stacked from insulated electrical sheet steel laminations and mounted
in supporting rings over insulated dovetailed guide bars. Axial compression of the stator
core obtained by clamping fingers, pressure plates and non-magnetic through-type
clamping boits, which are insulated from the core. The supporting rings form part of an
inner frame cage. This cage is suspended in the outer frame by a large number of
separate flat springs distributed over the entire core length. The flat springs are
tangentially arranged on the circumference in sets with three springs each, i.e. two
vertical supporting springs on both sides of the core and one horizontal stabilizing
spring below me core. The springs are so arranged and tuned that forced vibrations of
the core resulting from the magnetic field will not be transmitted to the frame and
foundation.
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The pressure plates and end portions of the stator core are effectively shielded against 233
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stray magnetic fields. The flux shields are cooled by a flow of hydrogen gas directly over no
assembly.
STATOR END SHIELDS
The ends of the stator frame are closed by pressure containing end shields. The end
shields feature a high stiffness and accommodate the generator bearings, shaft seals
and hydrogen coolers. The end shields are horizontally split to allow for assembly.
The end shields contain the generator bearings. This results in a minimum distance
between bearings and permits the overall axial length of the TE end shield to be utilized
{or accommodation of the hydrogen cooler sections. Cooler walls are provided in shield on
both sides of the bearing compartment for this purpose. One manhole in both the upper
and lower half end shield provides access to the end winding compartments of the
completely assembled machine.
Inside the bearing compartment, the bearing saddle is mounted and insulated from the
lower half end shield. The bearing saddle supports the spherical bearing sleeve and
insulates it from ground to prevent the flow of shaft currents.
The bearing oil is supplied to the bearing saddle via a pipe permanently installed in the
end shield and is then passed into the lubricating gap via ducts in the lower bearing
sleeve. The bearing drain oil is collected in the bearing compartment and discharged
from the lower half of the end shield via a pipe.
The bearing compartment is sealed on the air side with labyrinth rings. On the hydrogen
side, the bearing compartment is closed by the shaft seal and labyrinth rings. The oil for
the shaft seal is admitted via integrally welded pipes. The seal oil drained towards the
air side is drained together with the bearing oil. The seal oil drained towards the
hydrogen side is first collected in a gas and oil tight chamber below me Bearing 234
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compartment for defoaming and then passed via a siphon to the seal oil tank of the
hydrogen side seal oil circuit.
The static and dynamic bearing forces are directly transmitted to the foundation via
lateral feet attached to the lower half end shield. The feet can be detached from the end
shield. -since the end shields must be lowered into the foundation opening for rotor
insertion.
Stator Winding
Stator bars, phase connectors and bushings are designed for direct water cooling. In
order to minimize the stator losses, the bars are composed of separately
insulated/strands which are transposed by 540C in the slot portion and bonded
together with epoxy resins in heated molds. After bending the end turns are likewise bonded together with baked synthetic resin fillers.
The bars consist of hollow and solid strands distributed over the entire bar cross-section so
that good heat dissipation is ensured. At the bar ends, all the solid strands are jointly
brazed into a connecting sleeve and the hollow strands into a water box from which the
cooling water enters and exits via teflon insulating hoses connected to the annular
manifolds. The electrical CQ Cnection between top and bottom bars is made by a bolted
connection at the connecting sleeve.
The water manifolds are insulated from the stator frame permitting the insulation
resistance of the water-filled winding to be measured. During operation, the water
manifolds are grounded. 235
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Miscalastic High-voltage Insulation
High-voltage insulation is provided according to the proven Miscalastic system. With this
insulating system several halt-overlapped continuous layers of mica tape are applied to
the bars. The mica tape is built up from large area mica splitting which are sandwiched
between two polyester backed fabric layers with epoxy as an adhesive. The number of
layers, i.e., the thickness of the insulation depends on the machine voltage. The bars
are dried under vacuum and impregnated with epoxy resin which has very good
penetration properties due to its low viscosity. After impregnation under vacuum, the
bars are subjected to pressure, with nitrogen being used as pressurizing medium (VPI
process). The impregnated bars are formed to the required shape in MOULDS and
cured in an oven at high temperature. The high-voltage insulation obtained is nearly
void-free and is characterized by its excellent electrical, mechanical and thermal
properties in addition to being fully waterproof and oil-resistant. To minimize corona
discharges between the insulation and the slot wall, a final coat of semi conducting
varnish is applied to the surfaces of all bars within the slot range. In addition, all bars
are provided with an end corona protection to control the electric field at the transition
from the slot to the end winding and to prevent tie formation of creepage spark
concentrations.
Bar Support System
To protect the stator winding against the effects of magnetic forces due to load and to
ensure permanent firm seating of the bars in the slots during operation, the bars are
inserted with a side ripple springs, a hog-curing slot, bottom equalizing strip, and a top
ripple spring located beneath the slot wedge. The gaps between the bars. in the stator
end windings are completely filled with insulating material and cured after installation.
For radial support, the end windings are clamped to a rigid supporting of insulating
material which in turn is fully supported by 'the frame. Hot-curing conforming fillers
arranged between the stator bars and the support ring ensure a firm support of each 236
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individual bar against the support ring. The bars are clamped to the support ring with
pressure plates held by clamping bolts made form a high-strength insulating material.
The support ring is free to move axially within the stator frame so that movements of the
windings due to thermal expansions are not restricted.
The stator winding connections are brought out to six bushings located in a
compartment of welded r end. Current transformers or metering and relaying purposes
can be mounted on the bushings.
STATOR WINDING (Corona Protection)
To prevent potential differences and possible corona discharges between 'the insulation
and the slot wall, the slot sections of the bars are provided with an outer corona
protection. This protection consists of a wear-resistant, highly flexible coating of
conductive alkyd varnish containing graphite. 1 Stator bar (slot end)
2 High-voltage insulatation
3 Outer corona protection
4 Transition costing
5 End corona protection
6 Glass tape-epoxy protective layer 7
Stator her (end winding)
Fig. No. - 44 TYPICAL BUILDUP OF CORONA PROTECTION
At the transition from the slot to the end winding portion of the stator bars. a
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semiconductive coating is applied. On. top of this, several layers of semi-conductive end
corona protection coating are applied in varying lengths. This ensures uniform control of
the electric field and prevents the formation of corona discharge during operation and
during performance of high voltage tests.
A final wrapping of glass fabric tapes impregnated with epoxy resin serves as surface
protection.
ROTOR SHAFT
The high mechanical stresses resulting from the centrifugal forces and short-circuit call for
a high quality heat-treated steel. Therefore, the rotor shaft is forged from a vacuum cast
steel ingot. Comprehensive tests ensure adherence to the specified mechanical and
magnetic properties as well as a homogeneous forging.
The rotor shaft consists of an electrically active portion, the so-called rotor body, and the
two shaft journals. Integrally forged flange couplings to connect the rotor to the turbine
and exciter are located outboard of the bearings. Approximately two-thirds of the rotor
body circumference is provided with longitudinal slots which hold the field winding. Slot
pitch is selected so that the two solid poles are displaced by 180.
Due to the non-uniform slot distribution on the circumference, different moments of
intertia are obtained in the main axis of rotor. This in turn causes oscillating shaft
deflections at twice the system frequency. To reduce these vibrations, the deflections in
the direction of the pole axis and the neutral axis are compensated by transverse
slotting of the pole.
The solid poles are also provided with additional longitudinal slots to hold the copper
bars of the damper winding. The rotor wedges act as a damper windings in the area of the
winding slots. 238
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Rotor Winding
The rotor winding consists of several coils which are inserted into the slots and
seriesconnected such that two coil groups form one pole. Each coil consists of several
seriesconnected turns, eachof which consists of two half turns which are connected
by brazing in the end section.
The rotor winding consists of silver-bearing de-oxidized copper hollow conductors with
two lateral cooling ducts. L-shaped strips of laminated expoxy glass fiber fabric with
Nomex filler are used for slot insulation. The slot wedges are made of high-conductivity
material and extend below the shrink seat of the retaining ring. The seat of the retaining
ring is silver-plated to ensure a good electrical contact between the slot wedges and
rotor retaining rings. This system has long proved to be a good damper winding.
Retaining Rings
The centrifugal forces of the rotor end windings are contained by single-piece rotor
retaining rings. The retaining rings are made of non-magnetic high-strength steel in
order to reduce stray losses. Each retaining ring with its shrink-fitted insert ring is shrunk
into the rotor body in an overhung position. The retaining ring is secured in the axial
position by a snap ring.
Field Connection
The field current is supplied to the rotor winding through redial terminal bolts and two
semicircular conductors located in the hollow bores of the exciter and rotor shafts. The
field current leads are connected to the exciter leads at the exciter coupling with
Multikontakt plug-in contacts, which allow for unobstructed thermal expansion of the
field current leads.
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Hydrogen Cooler
The hydrogen cooler is a shell and tube type heat exchanger, which cools the hydrogen
gas in the generator. The heat removed from the hydrogen is dissipated through the
cooling water. The cooling water flows through the tubes, while the hydrogen is passed
around the finned tubes.
The hydrogen cooler is subdivided intoidentical sections, which are vertically mounted in
the turbine-end stator end shield. The cooler sections are solidly bolted to the upper half
stator-end shield, while the attachment at the tower water channel permits them to move
freely to allow for expansion.
The cooler sections are parallel connected on their watersides. Shut off valves are
installed in the lines before and after the cooler sections. The required cooling water
flow depends on the generator output and is adjusted by control valves on the hot water
side. Controlling the cooling water flow on the outlet side ensures an uninterrupted
water flow through the cooler sections so that proper cooler performance will not be
impaired.
The sleeve bearings are provided with hydraulic shaft lift oil during startup and turning
gear operation. To eliminate shaft currents, alt bearings are insulated from the stator
and base plate respectively. The temperature of the bearings is monitored with
thermocouples embedded in the lower bearings sleeve so that the measuring points are
located directly below the babbitt. Measurement and any required recording of the
temperature are performed in conjunction with the turbine supervision. The bearings
have provisions for fitting vibration pickups to monitor bearing vibrations.
Shaft Seals
The points where the rotor shaft passes through the stator casing are provided with a
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radial seal ring. The seal ring is guided in the seal ring carrier which is bolted to the seal
ring carrier flange Hid insulated to prevent the flow of shaft currents. The seal ring is
lined with babbitt on the shaft journal side. The gap between the seal ring and the shaft
is sealed with hydrogen side and air side seal oil. The hydrogen side seal oil is supplied
to the seal ring via an annular groove in the seal guide. Inside the seal ring, this seal oil
is fed to the hydrogen side annular groove in the seal ring and from there to the sealing
gap via several bores uniformly distributed on the circumference. The air side seal oil is
supplied to the sealing gap from the seal ring chamber via radial bores and the air side
annular groove in the seal ring. To ensure effective sealing, the seal oil pressures in the
annular gap are maintained at a higher level than the gas pressure within the generator
casing, the air side seal oil pressure being set to approximately the same level as the
hydrogen side seal oil pressure. The oil drained on the hydrogen side of the seat rings
is returned to the seal oil system through ducts below the bearing compartments. The
oil drained on the air side is returned to the seal oil storage tank together with the
bearing oil.
On the air side, pressure oil is' supplied laterally to the seal ring via an annular groove.
This ensure free movement of the seal ring in the radial direction.
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Cooling System
The heat losses arising in the generator interior are dissipated to the secondary coolant
(raw water, condensate. etc.) through hydrogen and primary water.
Direct cooling essentially eliminates hot spots and differential temperatures between
adjacent components which could result in mechanical stresses; particularly to the
copper conductors, insulation, rotor body and stator core.
Hydrogen Cooling Circuit
The hydrogen is circulated in the generator interior in a closed circuit by one multi-stage
axial-flow fan arranged on the rotor at the turbine end. Hot gas is drawn by the fan from
the air gap and delivered to the coolers, where it is recooled and then divided into three
flow paths after each cooler.
Flow Path - I
Flow path-1 is directed into the rotor at the turbine end below the fan hub for cooling of
the turbine end half of the rotor.
Flow Path - II
Flow path-11 is directed from the coolers to the individual frame compartments for cooling of the stator core.
Flow Path-Ill
Flow path-Ill is directed to the stator end winding space at exciter end through guide
ducts in the frame for cooling of the exciter end half of the rotor and of the core end 243
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portions.
The three (lows mix in the air gap. The gas is then returned to the coolers via the axialflow
fan.
The cooling water flow through the hydrogen coolers should be automatically controlled to
maintain a uniform generator temperature level for various toads and cold water
temperatures.
Cooling of Rotor
For direct cooling of the rotor winding, cold gas is directed to the rotor end windings at
the turbine and exciter ends. The rotor winding is symmetrical relative to the generator
center line and pole axis. Each coil quarter is divided into two cooling zones. The first
cooling zone consists of the rotor end winding and the second one of the winding
portion between the rotor body end and the mid-point of the rotor. Cold gas is directed
to each cooling zone through separate openings directly before the rotor body end. The
hydrogen flows through each individual conductor in closed cooling ducts. The heat
removal capacity is selected such that approximately identical temperatures are
obtained for all conductors. The gas of the first cooling zone is discharged from the coils
at the pole center into a collecting compartment within the pole area below the end
winding. From there, the hot gas passes into the air gap through pole face slots at the
end of the rotor-body. The hot gas of the second cooling zone is discharged into the air
gap at mid-length of the rotor body through radial openings- in the hollow conductors
and wedges.
Cooling of Stator Core
For cooling of the stator core, cold gas is admitted to the individual frame compartments
via separate cooling gas ducts. 244
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From these frame compartrnents. gas then flows into the air gap through slots in the
core where it absorbs the heat from the core. To dissipate the higher losses in the core
ends, the cooling gas slots are closely spaced in the core end sections to ensure
effective cooling. These ventilating ducts are supplied with cooling gas directly from the
end winding space. Another flow path is directed from the stator end winding space past
the clamping fingers between the pressure plate and core end section into the air gap. A
further flow path passes into the air gap along either side of the flux shield.
All the flows mix in the air gap and cool the rotor body and stator bore surfaces. The gas is
then returned to the coolers via the axial-flow fan. To ensure that the cold gas
directed to the exciter end cannot be directly discharged into the air gap. an air gap
choke is arranged within the range of the stator end winding cover and the rotor
retaining ring at the exciter end. ,
Primary Cooling Water Circuit In The Generator
The treated water used for cooling of the stator winding, phase connectors and
bushings is designed as primary water in order to distinguish it from the secondary
coolant (raw water, condensate, etc.). The primary water is circulated in a closed circuit
and dissipates the absorbed heat to the secondary cooling water in the primary water
cooler. The pump is supplied with hot primary water from the primary water tank and
delivers the water to the generator via the coolers. The cooled water flow is divided into
two flow paths as described in the following paragraphs.
Flow Path-1
Flow path-1 cools the stator windings. This flow path first passes to a water manifold on
the exciter end of the generator and from there to the stator bars via insulated hoses.
Each individual bar is connected to the manifold by a separate hose. Inside the bars,
the cooling water flow through hollow strands. At the turbine end, the water is passed 245
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through similar hoses to another water manifold and then returned to the primary water
tank Since a single pass water flow through the stator is used, only a minimum
temperature rise is obtained for both the coolant and the bars. Relative movements due
to different thermal expansions between the top and bottom bars are thus minimised.
Flow Path-11
Flow path-11 cools the phase connectors and the bushings. The bushings and phase
connectors consist of thick walled copper tubes through which the cooling water is
circulated. The six bushings and the phase connectors arranged in a circle around the
stator end winding are hydraulically interconnected so that three parallel flow paths are
obtained. The primary water enters three bushings and exits from the three remaining
bushings.
The secondary water flow through the primary water cooler should be controlled
automatically to maintain a uniform generator temperature level for various loads and
coldwater temperature.
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